1. Field of the Invention
The present invention relates to an automotive electric power supply apparatus provided with a belt-driven automotive dynamoelectric machine linked to an engine and an inverter unit for controlling the belt-driven automotive dynamoelectric machine.
2. Description of the Related Art
FIG. 8 is a conceptual diagram showing an automotive electric power supply apparatus using a conventional belt-driven automotive dynamoelectric machine, and FIG. 9 is a graph showing electric power output characteristics of the conventional dynamoelectric machine, the vertical axis representing output current in Amperes (A) and the horizontal axis representing rotational speed of the dynamoelectric machine in revolutions per minute (rpm).
In FIG. 8, a dynamoelectric machine 2 is a belt-driven dynamoelectric machine provided with: an armature winding 3 of a stator (not shown); and a field winding 4 of a rotor (not shown), the rotor being linked to a rotating shaft of an engine 1 by a belt (not shown). Here, the armature winding 3 is constructed by delta-connecting three phases of coil having four turns.
An inverter unit 5 is provided with: an inverter module 6 composed of a plurality of switching elements 8 and diodes 9 connected in parallel with each of the switching elements 8; and a capacitor 7 connected in parallel to the inverter module 6. This capacitor 7 has a role of smoothing the electric current flowing through the inverter module 6.
The inverter module 6 is constructed by forming element-diode sets each constituted by a switching element 8 and a diode 9 connected in parallel, connecting pairs of element-diode sets in series, disposing three such pairs in parallel, and sealing those components 8 and 9 integrally into a package. Each of the delta-connected end portions of the armature winding 3 is connected to a respective intermediate point between the switching elements 8 connected in series.
The switching operation of the switching elements 8 in the inverter module 6 is controlled by a control apparatus 10. When electric power is supplied, the dynamoelectric machine 2 operates as an electric starter motor to start the engine 1. After the engine 1 has started, the dynamoelectric machine 2 is driven to rotate by the engine 1 and operates as an alternator, generating a three-phase alternating-current voltage.
A first battery 11 constituting a driving electric power supply for the dynamoelectric machine 2 is connected in parallel to the inverter module 6. This dynamoelectric machine 2 is operated at high voltage (36 V, for example) by the first battery 11. Since the electrical machinery load mounted to an automotive vehicle is generally rated at 12 V, a 12-volt second battery 12 is also mounted. Thus, a direct-current-to-direct-current (DC-to-DC) converter 13 is connected in parallel to the inverter module 6 to enable the second battery 12 for driving the electric load to be charged.
In other words, during starting of the engine 1 by the dynamoelectric machine 2, it is necessary to increase the torque generated by the dynamoelectric machine 2, that is, to increase the amount of excitation current flowing to the armature winding 3. During operation with the second battery 12 for driving the electric load mounted to the automotive vehicle, loss in wiring becomes large, and in addition, the wiring itself is made larger in order to reduce wiring resistance. Thus, electric power transmission loss is reduced by increasing the voltage of the battery.
Next, operation of a conventional automotive electric power supply apparatus constructed in this manner will be explained.
First, the control apparatus 10 controls switching on and off of each of the switching elements 8 to generate three-phase alternating-current electric power from the direct-current electric power from the first battery 11. This three-phase alternating-current electric power is supplied to the armature winding 3 of the dynamoelectric machine 2, imparting a rotating magnetic field to the field winding 4 of the rotor and driving the rotor to rotate. Then, torque from the rotor is transferred to the engine 1 by means of the pulley and the belt (not shown), driving the engine 1 to rotate, that is, starting the engine 1.
Once the engine 1 has been started, torque from the engine 1 is transferred to the dynamoelectric machine 2 by means of the belt and the pulley- Thus, the rotor is driven to rotate, inducing a three-phase alternating-current voltage in the armature winding 3. Then, the control apparatus 10 controls switching on and off of each of the switching elements 8 to convert the three-phase alternating-current voltage induced in the armature winding 4 into a direct current. The first battery 11 is charged by the direct-current electric power rectified by the inverter unit 5. The direct-current electric power rectified by the inverter unit 5 is also converted to 12 V by the DC-to-DC converter 13 and supplied to the second battery 12.
Now, when the conventional dynamoelectric machine 2 is made to generate electricity in an alternator mode by switching each of the switching elements 8 off by means of the control apparatus 10, the electromotive force of the dynamoelectric machine 2 depends on the rotational speed of the rotor. In other words, when the rotational speed of the rotor of the dynamoelectric machine 2 is low, electric power generation in excess of a regulated voltage cannot be achieved in the alternator mode. Thus, when the rotational speed of the rotor is in a low-speed region, the dynamoelectric machine 2 must be made to generate electricity by an inverter mode.
In a conventional belt-driven automotive dynamoelectric machine, the torque transmission pulley ratio is around 2.5, and since the normal rotational speed region of a conventional engine is 1,200 to 3,000 rpm, the normal rotational speed region of the dynamoelectric machine 2 is 3,000 to 7,500 rpm.
Electric power generation by this dynamoelectric machine 2, as shown in FIG. 9, is switched from the inverter mode to the alternator mode when the rotational speed is in the vicinity of 7,000 rpm. Consequently, the dynamoelectric machine 2 generates electricity in the inverter mode over a large portion of its normal rotational speed region.
Electric power generation in this inverter mode is performed by switching each of the switching elements 8 by means of the control apparatus 10, and the faster the rotational speed of the dynamoelectric machine 2, the higher the rate at which the switching elements 8 are switched, that is, the higher the switching frequency. During electric power generation in the inverter mode, the electric current passing through the switching elements 8 is large compared to the electric current passing through the diodes 9 during electric power generation in the alternator mode. Thus, during electric power generation in the inverter mode, large currents are passed through the switching elements 8 continuously. Because the amount of heat generated in the switching elements 8 is great, the heat dissipation design of the inverter unit 5 is massive, water-cooled constructions, which have good cooling efficiency, being generally adopted to cool the inverter unit 5.
In an automotive electric power supply apparatus using a conventional belt-driven automotive dynamoelectric machine, because the dynamoelectric machine 2 generates electric power in the inverter mode, in which electric power generation loss is great compared to the alternator mode, over a large portion of its normal rotational speed region, some problems have been that a large-scale cooling construction is required for the inverter and also that the electric power generating efficiency of the dynamoelectric machine is reduced.
Furthermore, because it is necessary for the switching elements 8 to be controlled up to and in a high-speed rotation region of the dynamoelectric machine 2, thereby raising the switching frequency, another problem has been that the circuit configuration of the control apparatus 10 is complicated, leading to cost increases.